Wireless communication

ABSTRACT

The invention is directed to a method of synchronising transmission between two nodes in a wireless network. The method comprises the steps of obtaining an expected interference profile for each node; and agreeing a synchronised transmission schedule between the nodes, where the expected interference profile of each node meets predetermined criteria.

RELATED APPLICATION

This application is the full utility filing of U.S. provisionalapplication No. 60/447,646 filed on Feb. 14, 2003, from which thepresent application claims priority and which is incorporated herein byreference.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application is related to the following Provisional patentapplications filed in the U.S. Patent and Trademark Office, thedisclosures of which are expressly incorporated herein by reference:

-   -   U.S. patent application Ser. No. 60/446,617 filed on Feb. 12,        2003 and entitled “System for Coordination of Multi Beam Transit        Radio Links for a Distributed Wireless Access System” [15741]    -   U.S. patent application Ser. No. 60/446,618 filed on Feb. 12,        2003 and entitled “Rendezvous Coordination of Beamed Transit        Radio Links for a Distributed Multi-Hop Wireless Access System”        [15743]    -   U.S. patent application Ser. No. 60/446,619 filed on Feb. 12,        2003 and entitled “Distributed Multi-Beam Wireless System        Capable of Node Discovery, Rediscovery and Interference        Mitigation” [15742]    -   U.S. patent application Ser. No. 60/447,527 filed on Feb. 14,        2003 and entitled “Cylindrical Multibeam Planar Antenna        Structure and Method of Fabrication” [15907]    -   U.S. patent application Ser. No. 60/447,643 filed on Feb. 14,        2003 and entitled “An Omni-Directional Antenna” [15908]

U.S. patent application Ser. No. 60/447,644 filed on Feb. 14, 2003 andentitled “Antenna Diversity” [15913]

-   -   U.S. patent application Ser. No. 60/447,645 filed on Feb. 14,        2003 and entitled “Wireless Antennas, Networks, Methods,        Software, and Services” [15912]    -   U.S. patent application Ser. No. 60/451,897 filed on Mar. 4,        2003 and entitled “Offsetting Patch Antennas on an        Omni-Directional Multi-Facetted Array to allow Space for an        Interconnection Board” [15958]    -   U.S. patent application Ser. No. 60/453,011 filed on Mar. 7,        2003 and entitled “Method to Enhance Link Range in a Distributed        Multi-hop Wireless Network using Self-Cenfigurable Antenna”        [15946]    -   U.S. patent application Ser. No. 60/453,840 filed on Mar. 11,        2003 and entitled “Operating and Control of a High Gain Phased        Array Antenna in a Distributed Wireless Network” [15950]    -   U.S. patent application Ser. No. 60/454,715 filed on Mar. 15,        2003 and entitled “Directive Antenna System in a Distributed        Wireless Network” [15952]    -   U.S. patent application Ser. No. 60/461,344 filed on Apr. 9,        2003 and entitled “Method of Assessing Indoor-Outdoor Location        of Wireless Access Node” [15953]    -   U.S. patent application Ser. No. 60/461,579 filed on Apr. 9,        2003 and entitled “Minimisation of Radio Resource Usage in        Multi-Hop Networks with Multiple Routings” [15930]    -   U.S. patent application Ser. No. 60/464,844 filed on Apr. 23,        2003 and entitled “Improving IP QoS though Host-Based        Constrained Routing in Mobile Environments” [15807]    -   U.S. patent application Ser. No. 60/467,432 filed on May 2, 2003        and entitled “A Method for Path Discovery and Selection in Ad        Hoc Wireless Networks” [15951]    -   U.S. patent application Ser. No. 60/468,456 filed on May 7, 2003        and entitled “A Method for the Self-Selection of Radio Frequency        Channels to Reduce Co-Channel and Adjacent Channel Interference        in a Wireless Distributed Network” [16101]    -   U.S. patent application Ser. No. 60/480,599 filed on Jun. 20,        2003 and entitled “Channel Selection” [16146]

FIELD OF THE INVENTION

This invention relates to methods and apparatus for wirelesscommunication. The invention relates particularly, although notexclusively, to a wireless relay network.

BACKGROUND TO THE INVENTION

Wireless Community Area Networks (CANs) have been developed to provideaccess to the internet for wirelessly-enabled users. A CAN is a networkwith a size lying between a Wireless Local Area Network (LAN) and a WideArea Network (WAN). Thus a CAN may provide network access to usersdistributed over, say, a 1 km² area, such as a town centre or auniversity campus. A schematic diagram of a CAN is shown in FIG. 1.

The link 14 from the CAN to the user 12 often uses a cheap andwidely-available wireless standard, such as IEEE 802.11 set ofprotocols, often referred to for simplicity as ‘WiFi’.

Current CAN implementations, such as those installed at some USuniversity campuses (for example Carnegie Mellon University), useoff-the-shelf WiFi Access Points (APs) 10, connected to each other andto the broadband backbone 16 (and ultimately the internet) across a setof links 18 which is termed a ‘Distribution System’ (DS). This DS‘backhaul’ link usually uses a wired interface, most commonly based onIEEE 802.3 or ‘Ethernet’.

A wired DS is desirable from the point of view that it offers a reliablehigh-bandwidth/low latency path for onward transmission of data.However, the problem with this wired approach is that wires ofcommunications quality need to be provided to each AP, andinterconnected via wired switches/hubs/routers etc. In someenvironments, such as company or university campuses, this wiredinfrastructure may already be in place. However, in other environmentsthe installation and maintenance of this wired backhaul infrastructurecould be prohibitively expensive.

OBJECT TO THE INVENTION

The invention seeks to provide a method and apparatus for wirelesscommunication which mitigates at least one of the problems of knownmethods.

SUMMARY OF THE INVENTION

According to a first aspect of the invention there is provided a methodof synchronising transmission between two nodes in a wireless network,said method comprising the steps of obtaining an expected interferenceprofile for each node; and agreeing a synchronised transmission schedulebetween the nodes, where the expected interference profile of each nodemeets predetermined criteria.

Preferably, the expected interference profile is obtained by detectinginterference received at each node.

The interference profile may be characterised according to transmissionparameters, which may include time and frequency.

Each node may comprise a multiple beam antenna, and said transmissionparameters may further include the selected beam.

According to a second aspect of the invention there is provided a nodein a wireless network comprising: a memory for storing an expectedinterference profile of the node; a processor for determining where theexpected interference profile of the node meets predetermined criteria;and a transceiver for communicating with a second node to agree asynchronised transmission schedule according to the determination of theprocessor.

According to a third aspect of the invention there is provided wirelessnetwork comprising a plurality of nodes as described above.

According to a fourth aspect of the invention there is provided a methodof communication between two nodes in a wireless network, said methodcomprising the steps of: Obtaining an expected interference profile foreach node; Agreeing a synchronised transmission schedule between thenodes, where the expected interference profile of the or each node meetspredetermined criteria; and Effecting communication in accordance withthe synchronised transmission schedule.

According to a fifth aspect of the invention there is provided a signalfor agreeing a synchronised transmission schedule between a first and asecond node, said signal comprising a reference to a transmission slot,where the expected interference profile at a node meets predeterminedcriteria.

Advantageously, use of a wireless Distribution System (DS) avoids thehigh installation and maintenance costs of a wired DS.

Use of synchronised Transit Link Control allows the nodes to scheduletheir transmissions such that they can avoid interference to and fromeach other. It enables distant nodes to effectively coordinate theirtransmissions for the purposes of eliminating mutual interferencewithout needing explicitly to communicate directly with each other.

Additionally, it enables nodes to schedule their transmissions such thatthey can avoid interference from non-system interferers.

By dividing up the transmission bandwidth into a number of slotsaccording to a selection of transmission parameters, it provides agreater opportunity for nodes to find a transmission slot which issuitable for use.

Utilisation of the multiple degrees of freedom of wirelesscommunication, (e.g. beam, frequency, polarisation, burst time)mitigates interference and maximises system capacity.

Advantageously, this invention enables the sharing of carrierfrequencies within a wireless network using unspoken coordination.

Use of directivity within a Transit Node improves reach and minimisesinterference.

The method may be performed by software in machine readable form on astorage medium.

The preferred features may be combined as appropriate, as would beapparent to a skilled person, and may be combined with any of theaspects of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

An embodiment of the invention will now be described with reference tothe accompanying drawings in which:

FIG. 1 is a schematic diagram of a Community Area Network (Prior Art);

FIG. 2 is a schematic diagram of a Community Area Network according tothe present invention;

FIG. 3 is a schematic diagram of an installed Wireless Access andRouting Point (WARP) according to the present invention;

FIG. 4 is a schematic diagram of an example of a beam pattern of a WARPaccording to the present invention;

FIG. 5 shows an example of an Interference Table according to thepresent invention; and

FIG. 6 shows an example of a typical traffic loading for a 6 hop, 21WARP tree structure network according to the present invention.

DETAILED DESCRIPTION OF INVENTION

Embodiments of the present invention are described below by way ofexample only. These examples represent the best ways of putting theinvention into practice that are currently known to the Applicantalthough they are not the only ways in which this could be achieved.

Referring to FIGS. 2-6, there is shown an example of the presentinvention.

FIG. 2 shows a schematic diagram of a CAN. The network comprises anumber of Wireless Access and Routing Points (WARPs) 20, interconnectedby wireless links 22. This CAN therefore uses a wireless ‘DistributionSystem’ (DS). The WARPs perform a number of functions, including actingas access points (APs) for connection to mobile nodes (MNs) such asPDAs. The area of coverage of the access link (AL), which is the linkfrom the AP to a MN is shown schematically in FIG. 2 by the circles 26.The WARPs are wirelessly connected to each other and to the NetworkAccess Point (NAP) 24. The WARPs also act as Transit Nodes (TNodes), forwirelessly relaying information between WARPs and also between a WARPand a NAP. The NAP itself connects into a wired/fibre broadbandbackbone, which in turn is likely to be connected to the internet. Awireless link 22 (also referred to as a backhaul link) from one WARP (orTNode) to another, or between a WARP and a NAP is termed a Transit Link(TL), and the collection of TNodes and Transit Links is referred to as a‘Transit Network’ (or TNet). The CAN is used to transmit packets of dataand has the ability to forward packets towards their destination byhopping over wireless links through intermediate WARPs (acting asTNodes). FIG. 2 shows a single NAP within the CAN. However, in generalthere may be more than one NAP within the network.

The CAN shown in FIG. 2 uses wireless technology both for connection tomobile terminals (over the ‘Access Link’) and for backhaul to abroadband access point (in the wired network).

The CAN shown in FIG. 2 is in a mesh configuration. This is not the onlypossible configuration. Other configurations, such as a tree structure,may be more suitable for some networks.

FIG. 3 shows a schematic diagram of an installed WARP. The WARP 30 maybe fixed to a lamp post, utility pole, wall or other elevated mountingposition 32. The WARP provides an Access Link 34 to mobile nodes (MNs)36 and a Transit Link 38 to other WARPs or to a NAP (as shown in FIG.1). The Transit Link and the Access Link may use different wirelesstechnologies. For example the Transit Link may use 802.11a technology toprovide a 5 GHz link and the Access Link may use 802.11b technology toprovide a 2.4 GHz link. The coverage of the two links will not be thesame, as shown in FIG. 2, with the Access Link providing local coverage39 and the Transit Link operating over larger distances so as tocommunicate with other WARPs or NAPs.

In a preferred embodiment, the antenna on the WARP for the Transit Linkmay be a switched beam antenna and a schematic diagram of such a beampattern is shown in FIG. 4. The pattern comprises a number ofoverlapping beams and eight such beams are shown 41-48. The number ofbeams is not fixed at eight and there may be more beams or fewer beams.The WARP may have only one radio for the Transit Link, which means thatit can only transmit on one beam at any one time. This is beneficial asit reduces the cost of the WARP. Other antenna array processingtechniques such as transmit diversity and receive diversity may also bebeneficial.

In order to operate the Transit Network, a protocol is used whichoperates at a higher layer than the protocol which is used on theTransit Links (for example 802.11a) and at a lower layer than therouting protocol. This intermediate layer is referred to as the TransitLink Control (TLC) layer. The TLC layer is responsible for scheduling TLtransmissions on certain carrier frequencies, beams, time slots etc.This invention relates to a Synchronous Transit Link Control (S-TLC)layer. An alternative technique using an asynchronous approach isdescribed in a co-pending U.S. patent application.

For control purposes and to improve efficiency and accessibility, thetransmission bandwidth on the Transit Links is divided up intotransmission slots according to various transmission parameters. Thetransmission parameters may include time, frequency, beam, polarisationand any other suitable independent parameter. By this means thetransmission space is divided up into a multi-dimensional array ofpossible transmission slots. The length of the transmission slots (ortime slots) can be chosen according to the network requirements and theaccuracy of the clocks used. A long time slot may be beneficial in somecases as it requires a lower accuracy clock within the network nodes anda shorter time slot may be beneficial in some cases as it reduces thedelay before a signal can be sent (because the time to the start of thenext slot is reduced). The term ‘network node’ is used to refer to anynode within the Transit Network, including but not limited to WARPs andNAPs.

For S-TLC the clocks within the network nodes must be aligned. Theclocks may be exactly synchronised, (i.e. slot 1 is the same for allnodes) or alternatively time slot boundaries may be synchronisedalthough not absolute slot numbers (e.g. slot 1 on node 1 may correspondto slot 10 on node 2 but both slots start at the same time).

There are a number of techniques for aligning the clocks within thenetwork nodes and two techniques are described here:

1. Use of GPS (Global Positioning System): By incorporating a GPSreceiver into each node, each node will be synchronised to the centralGPS clock.

2. Distribution of time stamped packets: Data packets including timestamps are distributed between nodes and each node aligns its clock withany time stamp received from a faster running clock. This may beimplemented using the 802.11a Beacon Frame structure which alreadyincludes a time stamping function.

A second element of S-TLC is that each node has an Interference Table,as shown in FIG. 5. An interference table is a historical record ofwhich slots within the multi-dimensional array of transmission slots(described above) have tended to suffer from interference. Theinterference table therefore gives an indication of the expectedinterference in a given transmission slot. The interference table may besupplied to the node, but preferably the table is independently createdand maintained by each node within the network. Methods of measuringinterference are well known in the art. The interference table iscreated and maintained by periodically monitoring the receivedinterference levels and updating the table accordingly.

An example interference table is shown in FIG. 5. It is athree-dimensional array using the following parameters:

Beams, 50: 1-8 (although only data for beams 1, 2 and 8 shown)

Time slots, 51: 1-20

Carrier frequencies, 52: 1-8

These three parameters have been selected by way of example only. Anynumber of suitable transmission parameters can be used. Suitableparameters include, but are not limited to, beams, time slots, carrierfrequencies and polarisation.

In preparing an interference table, a repeat cycle must be selected,(e.g. 100 ms in this case, with this time being divided into 20 slots).This repeat cycle must be the same throughout the network. The tableshows the particular time slots on particular frequencies of each beamthat should not be used for transmission as historically they havesuffered from interference. The interference sources may betransmissions from other nodes within the network (as shown at 53) orsources outside the network (as shown at 54). The non-networkinterferer, which may be a nearby wireless LAN, may mean that a singlefrequency cannot be used at all for a particular beam. Preferably theinterference table is be compiled from averages of interference receivedover many repeat cycles. It is anticipated that interference tables willremain the same for periods of tens of minutes or longer.

It should be noted that each node may have a different interferencetable due to local interference effects. It is not necessary for a nodeto know the source of the interference it detects and records in itsinterference table. The node only needs to know that interference ispresent in order to avoid transmitting in the same slot. Theconsequences of transmitting in a slot where there is interferenceinclude:

-   -   i). A packet transmission is deferred (due to sensing of the        medium, and backoff, in the underlying Medium Access Control        layer)    -   ii). A packet is lost, because the interference was too high at        the receiver    -   iii). The packet was successfully sent, but at a lower data rate        than would otherwise have been possible in the absence of the        interference

All of these three outcomes listed are undesirable and should be avoidedif possible. Whilst a Synchronous TLC cannot totally guarantee thatinterference will be eliminated for each TL packet exchange, it cannevertheless significantly reduce the probability of such interferenceoccurring. It does this by enabling distant TNodes effectively tocoordinate their transmissions for the purposes of eliminating mutualinterference without needing explicitly to communicate directly witheach other for this purpose (the nodes may communicate directly witheach other for different purposes, such as authentication and routing).

As each node has its own interference table, it is necessary foradjacent nodes to agree some scheduled slots (referred to herein as‘skeds’) for transmission of packet data between them according to whenboth nodes have suitable slots within their interference tables. Asuitable slot is defined as one which meets preset criteria. Thesecriteria will preferably relate to the level of expected interference asdetermined from the interference table and an acceptable expectedinterference threshold may be defined. As each node may only have asingle radio for transmitting over a Transit Link, it will also benecessary for each node to ensure that they also are capable oftransmitting in that slot, (if there is only one radio, the node cannottransmit to more than one node at any one time). The scheduling ofinitial slots may be established on start up and subsequent slots may benegotiated during already agreed slots.

The scheduling of transmission slots may be for the purpose of settingup a new transmission link or for increasing the bandwidth of an alreadyexisting link. Scheduled slots may be agreed by an initiating nodesignalling to the proposed recipient with a proposal of a slot for ascheduled transmission. The recipient, referring to its own interferencetable, may refuse the slot or accept the slot. On refusal of the slotthe system may be established such that the initiating node or therecipient node proposes a new slot. The process can then be repeateduntil a mutually convenient slot is found.

A transceiver may be used to communicate to agree the scheduling ofslots. The term transceiver is used herein to mean any apparatus capableof transmitting and/or receiving information.

In the situation where clocks are aligned such that their time slotboundaries are coincident but where the time slot numbers are notnecessarily identical, it will be necessary for the nodes to confirmtheir respective slot numbering schemes during the negotiation for atransmission schedule (or skeds).

It is probable that any Transit Link will consist of multiple scheduledtransmission slots. In the situation with a multiple beam antenna, theseslots are all likely to use the same beam; however they may usedifferent frequencies or other parameters. A node should not set upmultiple transmission slots which greatly exceed the amount of data thatis likely to require forwarding, because this is likely to causeinterference variability to distant nodes. Interference variability maybe reduced by filling up unused slots with messaging or dummy data.Nodes should therefore take a long term view when establishing atransmission schedule with another node.

As described above, nodes should preferably monitor receivedinterference levels and update their interference tables accordingly.Additionally, in a preferred embodiment, nodes should also monitor whenpackets continually failed to be acknowledged during their regularscheduled transmissions (which have already been set up). When thisoccurs, the problematic scheduled transmission slot should be droppedand a new one established.

FIG. 2 shows a CAN having a mesh structure. This structure is not theonly possible structure and one possible alternative is a treestructure. If all or most of the network traffic is expected to passfrom the originating WARP, where the data was received via the AccessLink from a MN, to the broadband network via the NAP, then a treestructure may be more suitable.

FIG. 6 shows a typical traffic loading for a 6 hop, 21 WARP treestructure with traffic flow from the originating WARP to the NAP, viaother WARPs as required. The traffic loading at the extremities of thetree structure, WARPs A to F is very low with only 1 in 21 time slotsbeing utilised. Closer to the NAP, the traffic loading increases untilat WARP X, all 21 in 21 time slots are utilised. In this networkstructure a problem may arise in the delay between data arriving at anoriginating node A-E and being transmitted onwards to the next WARP G-L.If data arrives from a MN via the AL at WARP A in time slot 1, and thenext scheduled transmission to WARP G is not until time slot 21, it willbe necessary to wait for 20 time slots to pass before the data can passto WARP G. In order to mitigate this concern, WARP A and WARP G maynegotiate two (or more) transmission slots between themselves on theunderstanding that data is transmitted in any one of these slots. Inorder that the interference tables throughout the network are notaffected by the lack of transmission in the remainder of these slots(this is the effect of interference variability as described above),WARPs A and G may transmit dummy information (plus other signallinginformation etc) during the slots in which there is no data to send.

Although the above description describes implementation using 802.11wireless technology, this is not the only suitable technology. Any otherwireless technology could be used instead. Use of a widely availablewireless standard (such as 802.11) may provide additional benefits fromdesign and manufacturing economies of scale.

It will be understood that the above description of a preferredembodiment is given by way of example only and that variousmodifications may be made by those skilled in the art without departingfrom the spirit and scope of the invention.

1. A method of synchronizing transmission between two nodes in awireless distribution network, at least a first node having a multiplebeam antenna, said method comprising the steps of: dividing transmissionbandwidth into transmission slots; obtaining a multi-dimensionalinterference table giving an expected individual interference profilefor each respective node, said table showing particular time slots onparticular frequencies that should not be used for transmission, andwith respect to the at least first node showing particular time slots orparticular frequencies that should not be used for transmission for atleast two different beams of the multiple beam antenna of the at leastfirst node; and agreeing to a synchronized transmission schedule betweenthe nodes by an initiating node signaling to a proposed recipient aproposal of a slot for a scheduled transmission, where the expectedinterference profile of each node meets predetermined criteria andwherein both of the two nodes referring to their respective interferenceprofiles participate in the selection of a suitable transmission slotfor direct communication between said two nodes.
 2. A method ofsynchronizing transmission as claimed in claim 1, wherein the expectedinterference profile is obtained by detecting interference received ateach node.
 3. A method of synchronizing transmission as claimed in claim1, where the interference profile is characterized according to furthertransmission parameters.
 4. A method of synchronizing transmission asclaimed in claim 3, wherein the transmission parameters include timeand/or frequency and/or polarisation.
 5. A method of synchronizingtransmission as claimed in claim 4, wherein each node comprises amultiple beam antenna.
 6. A node in a wireless distribution networkcomprising: a multiple beam antenna; a memory for storing amulti-dimensional interference table giving an expected individualinterference profile of the node, said table showing particular timeslots on particular frequencies of at least two different beams of amultiple beam antenna of the node that should not be used fortransmission; a processor for determining where the expectedinterference profile of the node meets predetermined criteria; and atransceiver for communicating directly with a second node by use oftransmission slots to agree to a synchronized transmission schedulebetween said two nodes according to the determination of the processorand in case of the node initiating a communication, the node beingarranged for signaling to a proposed recipient a proposal of a slot fora scheduled transmission, and wherein the determination comprises theselection of a suitable transmission slot based on the respectiveinterference profiles of both nodes.
 7. A wireless network comprising aplurality of nodes as claimed in claim
 6. 8. A method of communicationbetween two nodes in a wireless distribution network, at least a firstnode having a multiple beam antenna, said method comprising the stepsof: dividing transmission bandwidth into transmission slots; obtaining amulti-dimensional interference table giving an expected individualinterference profile for each respective node, said table showingparticular time slots on particular frequencies that should not be usedfor transmission, and with respect to the at least first node showingparticular time slots or particular frequencies that should not be usedfor transmission for at least two different beams of the multiple beamantenna of the at least first node; agreeing to a synchronizedtransmission schedule between the nodes by an initiating node signalingto a proposed recipient a proposal of a slot for a scheduledtransmission, where the expected interference profile of each node meetspredetermined criteria, and wherein both of the two nodes referring totheir respective interference profiles participate in the selection of asuitable transmission slot for direct communication between said twonodes; and effecting communication in accordance with the synchronizedtransmission schedule.